Asha OTECseminar2

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OCEAN THERMAL ENERGY CONVERSION

SEMINAR REPORTSubmitted in partial fulfillment of

the requirements for the award of Degree of Master of Technology in Civil Engineering

(Environmental Engineering)of the University of Kerala

Submitted by

ASHA .C. RAJUM2, Environmental Engineering

Roll No: 091202

D E PA RT M E N T O F C I V I L E N G I N E E R IN G

COLLEGE OF ENGINEERING

TRIVANDRUM .

2010

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DEPARTMENT OF CIVIL ENGINEERING

COLLEGE OF ENGINEERING

TRIVANDRUM

Certificate

This is to certify that the interim seminar report entitled OCEAN THERMAL

ENERGY CONVERSION being submitted by ASHA .C. RAJU towards the partial fulfillment of the requirements for the award of Degree of Master of

Technology in Civil Engineering (Environmental ) of the University of Kerala is a

bonafide record of the work done by her under our supervision and guidance and that

this work has not been submitted elsewhere for a degree.

Guided by P.G. Professor

Mrs. LEA MATHEW Dr. V.SYAM PRAKASHLecturer Professor Department of Civil Engineering Department of Civil EngineeringCollege of Engineering College of EngineeringTrivandrum Trivandrum

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ACKNOWLEDGEMENT

I express my deep sense of gratitude to my guide Smt. Lea

Mathew, Lecturer, Department of Civil Engineering, College of Engineering, Trivandrum for her valuable guidance, constant

encouragement and creative suggestions offered during the course of this

seminar, and also in preparing this report.

I also express my sincere thanks to Dr.S.Sreekumar, Staff

Advisor, Asst .Prof. Sheeja A. K , Smt. Sindhu. P and Smt. Lea

Mathew , Seminar Co-ordinators, Dr. V. Syam Prakash, P.G. Professor

and Dr. S. Sheela, Head of the Department, Department of Civil

Engineering, College of Engineering, Trivandrum, for their kind co-

operation during the course of this work.

I would also wish to record my gratefulness to all my friends and

classmates for the help and support in carrying out this work successfully.

Last but not the least I express my heartful gratitude to Almighty

God who made this work flawless and trouble less

Asha .C. Raju

ABSTRACT

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Almost all countries are heavily dependent on fossil fuels to meet their

increasing energy needs for economic development. The high costs associated with fossil

fuels are placing an increasing strain on the economies of the countries. Within the last two

decades warnings of global destruction and climate change have become major issues. The

growth of environmental awareness and the rising demand for energy has urged researchers

to identify other sources of renewable energy. OTEC, or Ocean Thermal Energy

Conversion , is an energy technology that converts solar radiation to electric power. OTEC

systems use the ocean's natural thermal gradientthe fact that the ocean's layers of water

have different temperaturesto drive a power-producing cycle. As long as the temperature

between the warm surface water and the cold deep water differs by about 20C (36F), an

OTEC system can produce a significant amount of power.

The present study of Ocean Thermal Energy Conversion conducted by Martin et

al., (2008) and Hoshi et al., (2009) emphasized the point that the solution lies in using ocean

thermal energy as a resource rather than be unutilized. The first case study was concerning

the OTEC facility at Keahole Point on the Kona coast of Hawaii generates up to 1.2 mega

Watt-hours of electricity and 6 gallons per minute of desalinated water. The second was that

of an OTEC plant at Kumejima Island in southern part of Japan that utilizes not only ocean

thermal energy but also solar thermal energy as a heat source which is termed as Solar

Boosted Ocean Thermal Energy Conversion. SOTEC plant can potentially enhance the

annual mean net thermal efficiency up to a value that is approximately 1.5 times higher than

that of the conventional OTEC plant. The oceans are thus a vast renewable resource, with the

potential of about 10 13 watts of baseload power generation. Thus OTEC had been

demonstrated in recent studies as one of the environmental friendly methods for renewable

energy utilisation .

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CONTENTSLIST OF TABLESLIST OF ABBREVIATIONS

1. INTRODUCTION 1

1.1 GENERAL

2. MATERIALS AND METHODS 3

3. CASE STUDIES 5

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3.1. CASE STUDY 1 5

3.1.1. OPEN CYCLE OTEC 21

3.1.2 BENEFITS OF OTEC 10

3.1.3 APPLICATIONS OF OTEC 113.2 CASE STUDY 2 13

3.2.1. CONCLUSION 21

4. CONCLUSIONS 22

5. REFERENCES 23

LIST OF TABLESPage No:

Table 1 : Simulation results of 100-kWe OTEC and SOTEC operation 19

LIST OF ABBREVIATIONS

OTEC - Ocean Thermal Energy Conversion

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SOTEC - Solar Boosted Ocean Thermal Energy Conversion

DCC - Direct Contact Condenser

1. INTRODUCTION

1.1 General

Most of the countries are heavily dependent on fossil fuels to meet their increasing

energy needs. The relatively high costs associated with fossil fuels have encouraged research

into indigenous resources as an alternative source of energy. However, the potential of

indigenous resources is yet to be fully realised and exploited. The excessive use of fossil

fuels by industrialised countries has not only increased the carbon dioxide and other ozone-

depleting gases in the atmosphere, but has also contributed to global warming, sea-level rise

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and climate change The idea of using thermal energy from the ocean is not new. The high

costs associated with fossil fuels are placing an increasing strain on the economies in the

region. The growth of environmental awareness and the rising demand for energy has urged

researchers to identify other sources of renewable energy. OTEC, or ocean thermal energy

conversion, is an energy technology that converts solar radiation to electric power. OTEC

systems use the ocean's natural thermal gradientthe fact that the ocean's layers of water

have different temperaturesto drive a power-producing cycle. As long as the temperature

between the warm surface water and the cold deep water differs by about 20C (36F), an

OTEC system can produce a significant amount of power. The oceans are thus a vast

renewable resource, with the potential to help us produce billions of watts of electric power.

This potential is estimated to be about 10 13 watts of baseload power generation,

A significant factor in energy usage patterns in Pacific island countries has been

attributed to the availability of energy sources as early as the fifteenth century where

pollution control was not an issue. People have used what they have been able to get, be it

coal, oil, natural gas, wood, etc. However, within the last two decades warnings of global

destruction and climate change have become major issues. The growth of environmental

awareness and the rising demand for energy has urged researchers to identify other sources of

renewable energy. The heat stored in the ocean can be converted into electricity by means of

a process called Ocean Thermal Energy Conversion (OTEC), which uses the oceans natural

temperature gradient to drive a turbine connected to a generator which produces electricity.

Apart from electricity, there are other useful by-products from the OTEC process like fresh

water, chilled water and nutrient-rich water. To date there are basically three types of OTEC

systems developed to harness the ocean heat a closed-cycle, an open-cycle and a hybrid-

cycle. The economics of energy production have delayed the financing of permanent OTEC

plants.

There have been many periodic attempts to develop and refine OTEC technology

starting in the 1800s. In 1881, Jacques Arsene d'Arsonval , a French physicist , proposed

tapping the thermal energy of the ocean. It was d'Arsonval's student, Georges Claude who

actually built the first OTEC plant, in Cuba in 1930.The system generated 22 kW of

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electricity with a low- pressure turbine .In 1931, Nikola Tesla released "On Future Motive

Power" which covered an ocean thermal energy conversion system. Although initially

excited about the idea, Tesla ultimately came to the conclusion that the scale of engineering

required for the project made it impractical for large scale development. In 1935, Claude

constructed another plant, this time aboard a 10,000- ton cargo vessel moored off the coast of

Brazil . Weather and waves destroyed both plants before they could become net power

generators.

In 1956, French scientists designed a 3 MW plant for Abidjan , Cte d'Ivoire . The

plant was never completed, however, because large amounts of cheap oil became available in

the 1950s making oil fired power plants more economical In 1962, J. Hilbert Anderson and

James H. Anderson, Jr. started designing a cycle to accomplish what Claude had not; theyfocused on developing new, more efficient component designs. After working through some

of the problems in Claude's design they patented their new "closed cycle" design in 1967.

Although Japan has no potential OTEC sites it has been a major contributor to the

development of the technology, primarily for export to other countries. Beginning in 1970

the Tokyo Electric Power Company successfully built and deployed a 100 kW closed-cycle

OTEC plant on the island of Nauru . The plant, which became operational 1981-10-14,

produced about 120 kW of electricity; 90 kW was used to power the plant itself and the

remaining electricity was used to power a school and several other places in Nauru. This set a

world record for power output from an OTEC system where the power was sent to a real

power grid . (Hoshi et al., (2009)).

2. MATERIALS AND METHODS

The main objective of Ocean Thermal Energy Conversion (OTEC) is to turn the

solar energy trapped by the ocean into useable energy. This kind of energy is found in

tropical oceans where the water temperature differs from surface to deeper into the sea. On

the ocean surface it can be at least 20 0C hotter or cooler than the temperature at a deeper sea

level. It uses the temperature difference that exists between deep and shallow waters to run a

heat engine . As with any heat engine, the greatest efficiency and power is produced with the

largest temperature difference. This temperature difference generally increases with

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decreasing latitude, i.e. near the equator , in the tropics . Historically, the main technical

challenge of OTEC was to generate significant amounts of power, efficiently, from this very

small temperature ratio. Changes in efficiency of heat exchange in modern designs allow

performance approaching the theoretical maximum efficiency.

The Earth's oceans are continually heated by the sun and cover nearly 70% of the Earth 's

surface; this temperature difference contains a vast amount of solar energy which can

potentially be harnessed for human use. If this extraction could be made cost effective on a

large scale, it could provide a source of renewable energy needed to deal with energy

shortages , and other energy problems. The total energy available is one or two orders of

magnitude higher than other ocean energy options such as wave power , but the small

magnitude of the temperature difference makes energy extraction comparatively difficult andexpensive, due to low thermal efficiency .

A heat engine is a thermodynamic device placed between a high temperature

reservoir and a low temperature reservoir. As heat flows from one to the other, the engine

converts some of the heat energy to work energy. This principle is used in steam turbines and

internal combustion engines , while refrigerators reverse the direction of flow of both the heat

and work energy. Rather than using heat energy from the burning of fuel, OTEC power

draws on temperature differences caused by the sun's warming of the ocean surface

The only heat cycle suitable for OTEC, is the Rankine cycle , using a low-pressure

turbine. Systems may be either closed-cycle or open-cycle or hybrid cycle. Closed-cycle

engines use working fluids that are typically thought of as refrigerants such as ammonia .

Open-cycle engines use the water heat source as the working fluid . Hybrid systems use parts

of both open- and closed-cycle systems to optimize production of electricity and fresh water.

All OTEC plants require an expensive, large diameter intake pipe, which is submerged a

kilometre or more into the ocean's depths, to bring very cold water to the surface as shown in

fig: 2

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Fig :1 Pipes used for OTEC.Source : Martin et al.,(2008 )

3. CASE STUDIES

In this paper two case studies are being discussed. The first case study was

conducted by Martin et al.,(2008) concerning the OTEC facility at Keahole Point on the

Kona coast of Hawaii which generates up to 1.2 mega Watt-hours of electricity and 6

gallons per minute of desalinated water. The second conducted by Hoshi et al.,(2009)

discussed about an OTEC plant that utilizes not only ocean thermal energy but also solar

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thermal energy as a heat source which is termed as Solar Boosted Ocean Thermal Energy

Conversion. SOTEC plant can potentially enhance the annual mean net thermal efficiency up

to a value that is approximately 1.5 times higher than that of the conventional OTEC plant .

3.1 Case Study I

Fig :2 Land based OTEC facility at Keahole Point

Source : Martin et al.,(2008)

Figure 1 shows a land based OTEC plant installed at Keahole Point on

the Kona coast of Hawaii .. This plant was designed and operated for an output is 2.5 MW for

26 C warm surface water and a deep water temperature 6 C. A small fraction (10 percent)

of the steam produced was diverted to a surface condenser for the

production of desalinated water. The experimental plant is successfully

operating for six years. The highest production rates achieved were 2.35

MW (gross) with a corresponding net power of 1.2 MW and 6 gallons per minute

of desalinated water . These are world records for OTEC. Here electricity is

produced in a first-stage followed by water production in a second-stage,

to maximize the use of the thermal resource available to produce water

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and electricity. In the second-stage, the temperature difference available

in the seawater effluents from an OTEC plant (e.g., 12 C) is used to

produce desalinated water through a system consisting of a flash

evaporator and a surface condenser as shown in fig 3

Fig 3 Surface Condenser for Desalinated Water Production

Source: Martin et al.,(2008)

3.1.1 Open Cycle OTEC

The open cycle consists of the following steps: (i) flash evaporation of a fraction of

the warm seawater by reduction of pressure below the saturation value corresponding to its

temperature (ii) expansion of the vapor through a turbine to generate power; (iii) heat transfer

to the cold seawater thermal sink resulting in condensation of the working fluid; and (iv)

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compression of the non-condensable gases (air released from the seawater streams at the low

operating pressure) to pressures required to discharge them from the system. These steps are

depicted in Figure 4 and 5. In the case of a surface condenser the condensate (desalinated

water) must be compressed to pressures required to discharge it from the power generating

system. The evaporator, turbine, and condenser operate in partial vacuum ranging from 3

percent to 1 percent atmospheric pressure. This poses a number of practical concerns that

must be addressed. First, the system must be carefully sealed to prevent in-leakage of

atmospheric air that can severely degrade or shut down operation. Second, the specific

volume of the low-pressure steam is very large compared to that of the pressurized working

fluid used in closed cycle OTEC. Finally, gases such as oxygen, nitrogen and carbon dioxide

that are dissolved in seawater (essentially air) come out of solution in a vacuum. These gases

are uncondensable and must be exhausted from the system.

Fig 4. Open-Cycle OTEC Flow Diagram .

(Source: Martin et al.,(2008))

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Fig 5. Schematic diagram of Open-Cycle OTEC .

Source: Vega et al.,(2005)

In spite of the aforementioned complications, the Open cycle enjoyscertain benefits from the selection of water as the working fluid. Water, unlike ammonia, is

non-toxic and environmentally benign. Moreover, since the evaporator produces desalinated

steam, the condenser can be designed to yield fresh water. In many potential sites in the

tropics, potable water is a highly desired commodity that can be marketed to offset the price

of OTEC-generated electricity. Flash evaporation is a distinguishing feature of open cycle

OTEC. Flash evaporation involves complex heat and mass transfer processes. Here warm

seawater was pumped into a chamber through spouts designed to maximize the heat-and-

mass-transfer surface area by producing a spray of the liquid. The pressure in the chamber

(2.6 percent of atmospheric) was less than the saturation pressure of the warm seawater.

Exposed to this low-pressure environment, water in the spray began to boil. As in thermal

desalination plants, the vapor produced was relatively pure steam. As steam is generated, it

carries away with it its heat of vaporization. This energy comes from the liquid phase and

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results in a lowering of the liquid temperature and the cessation of boiling. Thus, as

mentioned above, flash evaporation may be seen as a transfer of thermal energy from the

bulk of the warm seawater to the small fraction of mass that is vaporized to become the

working fluid. Approximately 0.5 percent of the mass of warm seawater entering the

evaporator is converted into steam.

A large turbine is required to accommodate the huge volumetric flow rates of low-

pressure steam needed to generate any practical amount of electrical power. Although the last

stages of turbines used in conventional steam power plants can be adapted to Open Cycle-

OTEC operating conditions, existing technology limits the power that can be generated by a

single turbine module, comprising a pair of rotors, to about 2.5 MW. Unless significant effort

is invested to develop new, specialized turbines (which may employ fiber-reinforced plastic blades in rotors having diameters in excess of 100 m),increasing the gross power generating

capacity of a Open cycle plant above 2.5 MW. Condensation of the low-pressure working

fluid leaving the turbine occurs by heat transfer to the cold seawater. This heat transfer may

occur in a Direct Contact Condenser (DCC), in which the seawater is sprayed directly over

the vapor, or in a surface condenser that does not allow contact between the coolant and the

condensate. Direct Contact Condensers are relatively inexpensive and have good heat

transfer characteristics due to the lack of a solid thermal boundary between the warm and

cool fluids. Although surface condensers for OTEC applications are relatively expensive to

fabricate they permit the production of desalinated water. Desalinated water production with

a DCC requires the use of fresh water as the coolant. In such an arrangement, the cold

seawater sink is used to chill the fresh water coolant supply using a liquid-to-liquid heat

exchanger. Effluent from the low-pressure condenser must be returned to the environment.

Liquid can be pressurized to ambient conditions at the point of discharge by means of a pump

or, if the elevation of the condenser is suitably high, it can be compressed hydrostatically. For

a system that includes both the OC-OTEC heat engine and its environment, the cycle isclosed and parallels the Rankine cycle. Here, the condensate discharge pump and the non-

condensable gas compressor assume the role of the Rankine cycle pump.

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The analysis of the cycle yields (Figure 1):

Heat (added) absorbed from seawater (J/s) q w = m ww Cp (T wwi - T wwo)

Steam generation rate (kg/s) m s = q w/h fg

Turbine work (J/s) wT = m s (h3 - h 5) = m s T (h 3 - h 5s)

Heat (rejected) into seawater (J/s) q c = m cwCp (T cwo - T cwi)

where, m ww is the mass flow rate of warm water; Cp the specific heat; T wwi and T wwo are the

seawater temperature at the inlet and outlet of the heat exchanger; h fg the heat of evaporation;

and the enthalpies at the indicated points are given by h, with the subscript s referring to

constant entropy. The turbine isentropic efficiency is given by T. The subscript cw refers to

the cold water.

3.1.2 Benefits of OTEC

Conventional power plants pollute the environment more than an OTEC plantwould and, as long as the sun heats the oceans, the fuel for OTEC is unlimited and free

We can measure the value of an ocean thermal energy conversion (OTEC) plant and

continued OTEC development by both its economic and noneconomic benefits. OTEC's

economic benefits include these :

Helps produce fuels such as hydrogen, ammonia, and methanol

Produces baseload electrical energy

Produces desalinated water for industrial, agricultural, and residential uses

Is a resource for on-shore and near-shore mariculture operations

Provides air-conditioning for buildings

Provides moderate-temperature refrigeration

Has significant potential to provide clean, cost effective electricity for the future

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OTEC's noneconomic benefits, which help us achieve global environmental goals,

include these:

Promotes competitiveness and international trade

Enhances energy independence and energy security

Promotes international sociopolitical stability

Has potential to migrate greenhouse gas emissions resulting from burning fossil fuels

In small island nations, the benefits of OTEC include self-sufficiency, minimal

environmental impacts, and improved sanitation and nutrition, which result from the greater

availability of desalinated water and mariculture products.

3.1.3 Applications of OTEC

Ocean thermal energy conversion (OTEC) systems have many applications or uses.

OTEC can be used to generate electricity , desalinate water , support deep-water mariculture ,

and provide refrigeration and air-conditioning as well as aid in crop growth and mineral

extraction . These complementary products make OTEC systems attractive to industry and

island communities even if the price of oil remains low. OTEC can also be used to produce

methanol, ammonia, hydrogen, aluminum, chlorine, and other chemicals. Floating OTEC

processing plants that produce these products would not require a power cable, and station-

keeping costs would be reduced. Fig: 6 shows the overall process diagram of OTEC

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Fig 6 The overall process diagram of OTEC

Source: Martin et al.,(2008)

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3.2 CASE STUDY 2

Ocean thermal energy conversion (OTEC) is a power generation method that

utilizes small temperature difference between the warm surface water and cold deep water of

the ocean. The present case study at Kumejima Island in southern part of Japan describes the

performance simulation results of an OTEC plant that utilizes not only ocean thermal energy

but also solar thermal energy as a heat source. This power generation system was termed

SOTEC (solar-boosted ocean thermal energy conversion). In SOTEC, the temperature of

warm sea water was boosted by using a typical low-cost solar thermal collector. The results

show that the proposed SOTEC plant can potentially enhance the annual mean net thermal

efficiency up to a value that is approximately 1.5 times higher than that of the conventional

OTEC plant if a single-glazed flat-plate solar collector of 5000-m2

effective area is installedto boost the temperature of warm sea water by 20 K. The objective of the study was to

estimate the potential thermal efficiency and required effective area of a solar collector for a

100-kWe SOTEC plant, study was carried out under the ambient conditions at Kumejima

Island in southern part of Japan.

Ocean thermal energy conversion (OTEC) is a power generation method

where in the heat energy associated with the temperature difference between the warm

surface water and cold deep water of the ocean is converted into electricity .Considerable

research effort has been directed to the development of OTEC. The results of these studies

revealed that due to a small temperature difference (approximately 1525 K) between the

surface water and deep water of the ocean, the Rankine-cycle efficiency is limited to be only

35%. This results in a high cost of the electricity generated by an OTEC plant. In order to

improve the cycle efficiency, an ammoniawater mixture as the working fluid have been

developed and reported to have better thermal efficiency than the Rankine cycle at the same

temperature difference. However, it is evident that increasing the temperature difference

between the hot and cold heat sources is the most effective solution to improve the thermal

efficiency of a thermodynamic power generation cycle. In this study, an OTEC system was

described that utilizes not only ocean thermal energy but also solar-thermal energy; the latter

is used as a secondary heat source. A solar collector used in a residential application is

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simply installed to the conventional OTEC component. This power generation system is

termed as SOTEC (Solar-boosted Ocean Thermal Energy Conversion). The performance

simulation of a 100-kWe SOTEC plant with three typical low-cost solar-thermal collectors,

which increase the turbine inlet temperature of the working fluid, is carried out under the

actual weather and sea-water conditions at Kumejima Island in the southern part of Japan.

The simulation results of the SOTEC plant are discussed and compared with that of the

conventional OTEC plant.

Fig. 7. Schematics of conventional OTEC operation.

Source: Hoshi et al., (2009).

Figures. 7 and 8 show schematics of the conventional OTEC operation and the

proposed SOTEC operation, respectively; these figures show the general arrangement of the

heat exchangers, pumps, piping, turbine generator, and solar collector. In SOTEC, we present

the probable way to install solar collector into the cycle, as shown in Fig. 7. In Fig. 7, the

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warm sea water is pumped from the ocean surface and is heated by a solar collector; then, the

working fluid is indirectly heated and evaporated through the evaporator. In the simulation,

an ideal saturated Rankine cycle was assumed in order to determine the theoretical thermal

efficiency of the Rankine-cycle th.

Fig. 8. Schematics of SOTEC operation: Solar collector installed in warm-sea-water

Line


Fig. 9 shows the relationship between the theoretical thermal efficiency of the

Rankine-cycle th and the temperature difference T =T E -TC. Here, T E and T C are the

evaporation temperature and condensation temperature, The conventional OTEC has T

between 15 K and 25 K; as a result, the maximum theoretical thermal efficiency is

approximately th. = 8%. If a solar collector can additionally boost T E by 20 K, the

theoretical thermal efficiency of the SOTEC can be improved up to th. = 13%.

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Fig. 9 . Relationship between theoretical thermal efficiency of saturated Rankine cycle

th and the operating temperature range T of OTEC and SOTEC.


Fig. 10 shows a diagram of the proposed SOTEC plant. Its operation wasalternatively switched from OTEC to SOTEC by controlling valves. The heat transfer areas

of the evaporator and condenser were also ideally controlled to the optimal heat exchange

efficiency by controlling valves. Fig. 11 shows the monthly variation of the daytime net

Rankine cycle efficiency net of SOTEC operation compared with that of OTEC operation.

net of SOTEC is approximately three times higher than that of OTEC in every month.. net of

SOTEC was significantly affected by pumping powers.

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Fig :10 Diagram of the SOTEC plant .Source: Hoshi et al., (2009).

Fig. 11 . Daytime net Rankine-cycle efficiency net of OTEC and SOTEC


Figure 12 shows the monthly variations of the net power PN and the pumping

powers required for warm-sea-water PWS, cold-sea-water PCS, and the working-fluid PWF:

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Fig. 12(a) and (b) is for OTEC and SOTEC operations, respectively. These figures show a

breakdown of turbine-generated power PG = 100 kW.

Fig. 12 . Monthly variation of power of: (a) OTEC operation (b) SOTEC operation.


In SOTEC operation, the pumping power required for cold sea water was reduced to

approximately 30% of that in OTEC operation because the mass flow rate of cold sea water

was reduced in SOTEC operation due to an increase in the Rankine-cycle efficiency R by

solar boosting. Consequently, the net power of SOTEC was larger than that of OTEC. The

pumping powers required for warm seawater and the working fluid were also slightly

reduced in SOTEC. These resulted in the above-mentioned increase in the net efficiency of

SOTEC.

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Table: 1 Simulation results of 100-kWe OTEC and SOTEC operationSource: Hoshi et al., (2009).

Table 1 lists important simulated values of 100-kWe OTEC and SOTEC

operations under the annual mean daytime condition at Kumejima Island. In this table, the

result obtained for the case of 40-K solar boost is added for comparison. net of OTEC

daytime operation was 2.3%, while that of SOTEC was 6.3% and 9.5% corresponding to 20-

K and 40-K solar boost, respectively. The required effective area of the solar collector (ASC)

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of the flat-plate solar collector for the 40-K solar boost is 5333 m 2, which is 893 m 2 larger

than that of the 20-K solar boost because of the collector efficiency drop from c=63% to

48%. In contrast, ASC of the evacuated-tube collector for the 40-K solar boost is

approximately 100 m 2 less than that for the 20-K solar boost because the collector efficiency

was retained at c = 68% even for the 40-K boost.

Fig. 13 . Net Rankine-cycle efficiency net of SOTEC plant and OTEC plant


Fig. 13 shows the simulation results for the annual hourly variation of the net

Rankine-cycle efficiency net of the SOTEC plant and the conventional OTEC plant at

Kumejima Island for the 20-K solar boost. The SOTEC plant was selectively operated in the

SOTEC mode during daytime only if the thermal efficiency of SOTEC operation exceeded

that of OTEC operation. In Fig. 12, net of the SOTEC plant fluctuated up to 9% due to the

change in solar gain, while net of OTEC was within the range between approximately 1%

and 3%. Further, the net power of SOTEC fluctuated within the range between

approximately 70 kWe and 150 kWe; this is not shown in Fig. 11. The annual total SOTEC

operation time at Kumejima Island was estimated to be 2867 h (approximately 120 days), so

that the annual mean net Rankine-cycle efficiency of the SOTEC plant was finally estimated

to be approximately 3%. This value is equivalent to 145% of that of the conventional OTEC

plant. The present simulation results indicate that the SOTEC operation can potentially

increase the efficiency of OTEC by means of a combination of low-cost solar collectors. 40-

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K boost by the solar collector enhances net thermal efficiency up to several times higher than

the conventional OTEC in daytime. In terms of cost estimation for the SOTEC, the cost of a

flat-plate solar collector would be lower than that of other solar collectors to achieve a solar

boost upto 20 K and that the evacuated-tube collector would be cost-effective for the SOTEC

with 40-K solar boost. The price of evacuated-tube collectors is being dramatically be

reduced due to market expansion and the development of mass production technology. The

development of a mass-production technology for large-area solar collectors will help reduce

the cost of SOTEC.

3.2.3 Conclusion

A solar-boosted ocean thermal energy conversion (SOTEC) system was

proposed and performance simulation was carried out. The results reveal that the installation

of a solar collector enhances the thermal efficiency of an OTEC plant, particularly in daytime

operation. Net thermal efficiency of SOTEC operation with 20-K solar boost is 2.7 times

higher than that of OTEC operation under the daytime conditions at Kumejima Island. This

results in approximately 1.5-times higher annual net thermal efficiency than the conventional

OTEC plant. For future studies, the authors intend to perform more precise simulation

including other thermodynamic cycles and estimate the practical cost of SOTEC plant. Thedevelopment of an advanced OTEC system will become increasingly important and

promising with a long-term upward trend in the prices of oil and fossil fuels .

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4. CONCLUSIONS

The present study emphasises the importance of using ocean thermal energy

as a resource rather than be unutilized. Conventional power plants pollute the environment

more than an OTEC plant would and as long as the sun heats the oceans, the fuel for OTEC

is unlimited and free.

1. The main objective of Ocean Thermal Energy Conversion (OTEC) is to turn the solar energy

trapped by the ocean into useable electric energy of 10 13 watts of base load power generation

2. It also desalinate water , support deep-water mariculture, and provide refrigeration and air-

conditioning as well as aid in crop growth and mineral extraction .

3. OTEC facility at Keahole point on the Kona coast of Hawaii generates upto 1.2 mega Watt

hours of electricity and 6 gallons per minute of desalinated water.

4. SOTEC (Solar Boosted Ocean Thermal Energy Conversion) utilizes not only ocean thermal

energy but also solar thermal energy as a heat source.

5. Net thermal efficiency of SOTEC operation with 20-K solar boost is 2.7 times higher than

that of OTEC operation under the daytime conditions at Kumejima island.

6. SOTEC plant can potentially enhance the annual net thermal efficiency upto a value that is

approximately 1.5 times higher than that of the conventional OTEC plant.

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http://www.nrel.gov/otec/desalination.htmlhttp://www.nrel.gov/otec/mariculture.htmlhttp://www.nrel.gov/otec/refrigeration.htmlhttp://www.nrel.gov/otec/refrigeration.htmlhttp://www.nrel.gov/otec/mineral_extraction.htmlhttp://www.nrel.gov/otec/desalination.htmlhttp://www.nrel.gov/otec/mariculture.htmlhttp://www.nrel.gov/otec/refrigeration.htmlhttp://www.nrel.gov/otec/refrigeration.htmlhttp://www.nrel.gov/otec/mineral_extraction.html


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5. REFERENCES

1) Hoshi, A., Yamada, N., Yasuyuki, I.,(2009) Performance simulation of solar-boosted

ocean thermal energy conversion Plant, Journal of Renewable Energy, Vol 34, pp

1752-1758

2) Martin, L, L., Moore ,F, P., (2008) A nonlinear nonconvex minimum total heat

transfer area formulation for ocean thermal energy conversion (OTEC) systems

Journal of Applied Thermal Engineering, Vol 28, pp 10151021

3) Vega, L, A., Nelson, P, M., (2005) Ocean Thermal Energy Conversion: Electricity

and Desalinated Water Production, Journal of Renewable Energy, Vol : 14, pp 785 -

794

4) Wikipedia < http://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion >

5) http://
http://en.wikipedia.org/wiki/Ocean_thermal_energy_conversionhttp://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion

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